Various air-conditioning apparatuses including injection circuits have hitherto been proposed. One of such apparatuses is a “refrigeration apparatus including a liquid injection circuit in which a compressor, a condenser, a receiver, a pressure reducing device, and an evaporator are sequentially connected in a loop and in which a liquid refrigerant is supplied from the receiver to the compressor, wherein the liquid injection circuit is provided with a capillary tube and a flow control valve and the flow regulating valve adjusts the amount of injection on the basis of the discharge temperature of the compressor” (see, for example, Patent Literature 1). This refrigeration apparatus is designed to detect the discharge temperature of the compressor, change the opening degree of the flow regulating valve in accordance with the detected temperature, and control the injection flow rate.
There is also a “heat pump air conditioner for cold climate regions in which at least a heat source side heat exchanger, a pressure reducing device, a use side heat exchanger, and a scroll type compressor are sequentially connected to form a refrigeration cycle, and a refrigerant circuit that injects a liquid refrigerant into a compression mechanism in the scroll compressor is provided” (see, for example, Patent Literature 2). This heat pump air conditioner is designed to perform injection to control the discharge temperature of the compressor even in a case where the circulation path in the refrigeration cycle is reversed (to switch between the cooling and heating operations).
There is also an “air-conditioning apparatus including a compressor, a plurality of indoor heat exchangers, and a plurality of outdoor heat exchangers; a plurality of outdoor-unit-side flow path switching units each connected to a first connecting port of one of the outdoor heat exchangers, a discharge port of the compressor, and a suction port of the compressor, each outdoor-unit-side flow path switching unit switching a refrigerant flow path to a refrigerant flow path through which a refrigerant flows from the discharge port of the compressor to the first connecting port of the corresponding one of the outdoor heat exchangers or to a refrigerant flow path through which a refrigerant flows from the first connecting port of the corresponding one of the outdoor heat exchangers to the suction port of the compressor; a plurality of indoor-unit-side flow path switching units each connected to a first connecting port of one of the indoor heat exchangers, the discharge port of the compressor, and the suction port of the compressor, each indoor-unit-side flow path switching unit switching a refrigerant flow path to a refrigerant flow path through which a refrigerant flows from the discharge port of the compressor to the first connecting port of the corresponding one of the indoor heat exchangers or to a refrigerant flow path through which a refrigerant flows from the first connecting port of the corresponding one of the indoor heat exchangers to the suction port of the compressor; a connecting pipe that connects second connecting ports of the outdoor heat exchangers to second connecting ports of the indoor heat exchangers; a pressure reducing device disposed in the connecting pipe; and an injection circuit whose one end is connected to the connecting pipe between the pressure reducing device and the indoor heat exchangers and whose other end is connected to a compression process in the compressor, the injection circuit injecting a refrigerant flowing through the connecting pipe into the compression process in the compressor” (see, for example, Patent Literature 3). This air-conditioning apparatus is capable of performing injection in the cooling, heating, or cooling and heating mixed operation, and generates an intermediate pressure to perform injection during heating.